James Hewitt – the “love rat” who brazenly bedded Princess Diana under Prince Charles’ nose – is now revealing just how big a slimeball he really is, claiming in a new TV bio that his tragic royal lover was a tigress in the sack.

The former military officer also thumbs his nose at Charles and suggests the Prince of Wales was grateful his wife was being sexually satisfied.

Hewitt also discloses portions of 64 intimate love letters Diana wrote him in which she gushes about how she “screams [with] delight” and gets “demented” when she thinks about him.

Hewitt’s details about his five-year affair with Diana – from 1986 to 1991 – has Buckingham Palace apoplectic with rage and friends of the late princess seething with disgust.

“All those tags have been used before, but come nowhere near to summing up this disgusting creature.”

The Sun joined the media frenzy by sending out a reporter dressed in a rat suit to greet Hewitt at his home.

Hewitt spills the never-before-heard details of his royal affair in a documentary about his life called “Confessions of a Cad,” which will be aired Thursday on Britain’s Channel 4.

Right from the start, the 45-year-old tattletale makes no apologies for his boorish behavior.

“I never wanted to be a cad but I guess I am, so, if you’re handed a bunch of lemons, make some lemon juice,” he writes.

Proving just how much of a kiss-and-tell snake he is, Hewitt then goes on to recount salacious details of his sex life with Princess Diana, and says he’ll try to sell some of her love letters – notes she once begged him to destroy – on eBay.

“Boy, oh boy, does the earth shake when I get a letter from my desert friend, screams of delight, tears, you name it. Demented female on the loose, that’s for sure.”

In another, dated the day the war began, she writes: “So it’s started and I sit here glued to the TV and radio hoping you’re OK. I’m extremely worried and pray the war will be over soon. Your letter was such a joy.”

In the documentary, Hewitt is asked how many women he’s been involved with in his life. Puffing up like a peacock, he wisecracks, “This week? This year?”

He later jokes that the number of lovers he’s bedded exceeds the average number of runs scored by the English cricket team in a season – from 2,000 to 4,000.

The documentary trails Hewitt during the past six months of his life and includes footage of him on a recent trip to the United States, where he failed to sell Diana’s letters for $15 million.

Channel 4 says Hewitt was paid a modest fee for his cooperation and friends say he’s using the show as another attempt to drum up interest in the letters. Hewitt quips: “We’re doing this program because Im a complete s- – -, and we’re trying to make me less of a s- – -. And it’s not working!”